Astronomers Get Gnarly Over Planetary Definitions

The world’s astronomers are gathered at the International Astronomical Union (IAU) General Assembly in Prague to ratify new definitions for “planets” and the smaller “solar system bodies” such as comets and asteroids that make up our Solar System. If the new definitions from the IAU are accepted, our solar system will have 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of “plutons” (Pluto-like objects) and Ceres. Somewhat confusingly, Pluto itself is not a pluton, and remains a planet.

The new definitions are needed thanks to the increasing number of odd-sized celestial objects that astronomers are turning up. These objects present a challenge to our historically based definition of a “planet.” For example, how large, and how round should an asteroid be before it becomes a planet; and where is the upper limit? How large can a planet be before it becomes a brown dwarf or a star?

The IAU has been the sole arbiter of planetary and satellite nomenclature since its inception in 1919. The world’s astronomers, under the auspices of the IAU, have had official deliberations on a new definition for the word “planet” for nearly two years. IAU’s Executive Committee, led by President Ron Ekers, formed a Planet Definition Committee (PDC) comprised by seven persons who were astronomers, writers, and historians with broad international representation. This group of seven convened in Paris in late June and early July 2006. They culminated the two year process by reaching a unanimous consensus for a proposed new definition of the word “planet.”

“In July we had vigorous discussions of both the scientific and the cultural/historical issues, and on the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus. But by the end of a long day, the miracle had happened: we had reached a unanimous agreement,” said Owen Gingerich, the Chair of the PDC. The part of “IAU Resolution 5 for GA-XXVI” that describes the planet definition, states “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

According to the new draft definition, two conditions must be satisfied for an object to be called a “planet.” First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation. “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet,” said PDC Member, Richard Binzel.

Assuming the resolution is passed; the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. The name 2003 UB313 is provisional, as a “real” name has not yet been assigned to this object.

The new resolution also seeks to establish Plutons as a new category of planet. Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets and have orbits that are far from being perfectly circular. All of these distinguishing characteristics for plutons suggest a different origin from the classical planets.